Contour ploughing helps in soil conservation ?

When I work with schoolkids on conservation topics, here's how I explain it. Contour farming and contour terraces reduce erosion by reducing the speed at which water flows downhill. Think of the way stair steps allow us to descend a steep incline in a slow and safe manner and you get the idea. Try to descend a steep incline without steps and pretty soon you're going so fast that you cannot control your footsteps.